1) Field Of The Invention
The present invention relates to biaxial stretch-oriented, and heat-set film made of thermoplastic which has at least on one of its surfaces a surface roughness (R.sub.a) of greater than 0.003 .mu.m (microns). The invention also relates to a process to manufacture such a film.
2) Prior Art
It is known that very clear films made of thermoplastic show a particular undesirable tendency of blocking when wound on rolls due to their very smooth surfaces. German Patent DE-A-36 20 511 describes a solution to this problem whereby inert additives in extremely fine form are added to the thermoplastic (in this case polyester). Due to these additives, a micro roughness appears on the film surface which is responsible for an extremely thin air layer between the individual film layers of a film roll and provides for spacing between the individual film layers and, therefore, hinders blocking of the film.
In these films, the inert particles responsible for the surface features are distributed over the entire film resulting in a certain cloudiness of the film. To minimize the cloudiness, multi-layer structures are formed to improve the transparency of said polyester films, whereby the core layers are free of inert particles and only the outer layers contain inert particles having a specific order of magnitude. Said films are described in the European Patent 0 260 258.
However, it is desirable for certain usages, especially in the reprographic sector, to produce films with still further improved transparency whereby all particle addition is avoided, but where a certain sliding ability (anti-blocking) is guaranteed for handling and cutting purposes.